Field of the Invention (Technical Field)
The present invention relates epitaxial growth of a layer of a hexagonal material, such as gallium nitride (GaN) or other group III-nitride (III-N) semiconductors, on a substrate whose crystalline alignment is formed by the ion-beam assisted deposition (IBAD) texturing process. In one embodiment, IBAD textured layers are used to prepare biaxially aligned thin films or substrates which are single-crystal-like in nature. These IBAD thin films or templates support subsequent deposition of optional epitaxial buffer layers followed by GaN or III-N epitaxial growth. An electronic component that includes III-N epitaxy on an ion-beam textured layer with intermediate epitaxial buffer layers on top and a method of forming the same are disclosed.
An embodiment of the present invention is an ion beam assisted deposition (IBAD) texturing process for biaxially aligned films as templates for GaN epitaxy. The IBAD process enables low-cost, large-area, flexible metal foil substrates to be used as potential alternatives to single-crystal sapphire and silicon for electronic devices. Epitaxial GaN films are grown by the MOCVD process on these engineered flexible substrates, which enables scaled-up roll-to-roll, sheet-to-sheet, or similar fabrication processes to be used. GaN films having a thickness of several microns on polycrystalline metal foils that have in-plane and out-of-plane alignment of less than 1° have been manufactured. The epitaxial GaN films on polycrystalline metal foil are used as a template layer to make multi-quantum well light emitting diode (LED) structures and have successfully demonstrated electro-luminescence. These are the first LED devices fabricated directly on metal foil, and can be scaled up using a roll-to-roll process.
Background Art
Note that the following discussion may refer to a number of publications and references. Discussion of such publications herein is given for more complete background of the scientific principles and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Light-emitting diodes (LEDs) are revolutionizing the way the world is implementing lighting at the start of the 21st century. Not only are LEDs more efficient light sources, but they have the ability to be implemented in many different forms compared to other light sources and to have their spectrum adjusted for application as well as modified in time. However, the greatest barrier still holding back LEDs from completely replacing incandescent and fluorescent lighting is cost of the LED luminaire systems. Although LED lighting has made great strides in penetrating the lighting market, it is currently mostly focused in the niche high-end lighting space and still far from a mainstream application in commercial lighting where it is hard to compete in cost with simple fluorescent tubes. For LEDs to dominate the whole lighting market the cost of LED lighting will still have to come down by several orders of magnitude. This is in spite of the fact that the LED chip and package costs have already come down incredibly by several orders of magnitude in the past decade. The cost of packaged LEDs today can even be less than $0.50/klm, compared to an average LED package price of $50/klm a decade ago. There is still room to go in reducing cost by another factor of 2 or 3 using current fabrication techniques. To go significantly further in cost reduction one has to tackle the critical issue of scale in manufacturing of the LED chips and packages as well as reduction in the cost of other parts of the luminaire system. The packaged LEDs are used as surface mounted devices (SMD) and typically implemented with a pick-and-place (P&P) technology in lighting devices. P&P machines are automated ways of mounting SMDs mechanically. Eliminating SMD's and P&P would simplify the LED luminaire considerably and reduce costs. The way semiconductor industry does scale up of semiconductor chips is to increase the substrate size incrementally from 2-inch to 4-inch and now going to 6-inch single-crystal wafers. Most of blue LED production today is done using a GaN platform on sapphire. High-quality epitaxial GaN is deposited usually by metal-organic chemical vapor deposition on sapphire (MOCVD, sometimes called OMVPE or organo-metallic vapor phase epitaxy) and then used as a platform for subsequent deposition of epitaxial device structures.
GaN and related group III-N materials are used for numerous applications, including light emitting diodes (LEDs), laser diodes (LDs), and transistor devices such as high-electron mobility transistors (HEMTs). The vast majority of today's GaN layers are deposited epitaxially on single-crystal substrates such as sapphire, silicon, silicon carbide, or gallium nitride. However, single-crystal substrates are typically rigid, expensive, and readily available in diameters of only less than 100 mm, except for silicon wafers. An exception to single-crystal substrates is the development of ion-beam assisted deposition (IBAD) of single-crystal like thin films on flexible substrates. In the last decade, IBAD texturing of thin films on flexible metal has been notably been developed for long lengths of superconducting crystals for electrical wire applications.
When typically growing GaN using MOCVD (metal-organic chemical vapor deposition) on single-crystal substrates that are not native (i.e. heteroepitaxially), a two-step deposition process is used whereby an initial GaN nucleation layer (NL) is deposited at a relatively low temperature (500-600° C.) to facilitate GaN nucleation and evolution. In the second step, the fully coalesced epitaxial GaN layer is then grown at a higher temperature (>1000° C.) to obtain device quality GaN material on top of the NL. Limiting the growth of GaN to lattice-matched single-crystal substrates reduces the number of practical substrates to sapphire (Al2O3), SiC, and bulk-GaN, which can be expensive and unavailable in large sizes. More recently Si has been developed as a single crystal substrate for GaN epitaxy and is becoming more widespread. Despite the adoption of Si as a potential alternative substrate to sapphire, direct growth of GaN on metal and other substrates is desired for practical applications that need large area or flexible substrates. Thus far it has not been possible to grow single-crystal GaN directly on metal or other non-oriented substrates due to the lack of epitaxial registry. GaN on metal or other non-oriented substrates has been achieved by transfer of the grown epitaxial GaN layer onto the foreign substrate, or by transferring an oriented film such as graphene and growing GaN on top of the graphene.
Previous IBAD texturing in fluorites was not developed, not easy to work with, and the IBAD texture widths were more than 15° in-plane FWHM. Thus IBAD (111) was not thought to be of sufficient quality to produce high-quality semiconductor materials with in-plane alignment of <1°. The best semiconductor Si results on (111) IBAD had in-plane texture FWHM of >10° and out-of-plane texture 1.5°. Thus typical semiconductor materials on IBAD are of inferior quality for devices and cannot compete with semiconductors on single crystal substrates. Good quality LED and other devices have not been produced. Several previous attempts to make GaN devices have been unsuccessful, since the materials have not been of high enough quality in terms of crystalline perfection and carrier mobility.